In pumping oil from underground reservoirs, it is conventional to utilize a motor driven walking beam to vertically reciprocate a string of sucker rods to actuate a downhole pump. The upper end of the string of sucker rods includes a polished rod which reciprocates through a stuffing box. It is desirable to measure the load in the string of sucker rods for monitoring the operation of the pump and the axial loads in the sucker rods.
One apparatus for measuring the loads in the string of sucker rods is the use of a strain gauge transducer, such as shown in U.S. Pat. No. 3,965,736, which utilizes a split cylinder clamped around the polished rod which has strain gauges bonded thereto which measures the strain of the polished rod. However, it is very rare that the polished rod will have only an axial load. Usually there is a slight bending of the polished rod during its reciprocation up and down. Bending of the polished rod will create positive and negative strains on opposite sides of the polished rod. A split cylinder type of transducer will generally measure both the bending strains and the axial strain in the polished rod and will not give an accurate measurement of only the axial strain in the polished rod, which is the property desired to be measured. Furthermore, it is also very rare that the polished rod size is exact. A split cylinder type of transducer only contacts an undersized or oversized polished rod at a minimum contact area and is subject to slippage under heavy rod loads which causes further errors in measuring the load in the rod string.
The present invention provides an improved polished rod load transducer which overcomes the problems of the prior art by providing a configuration which cancels out the effects of bending on the polished rod and therefore measures only the axial or load strain in the polished rod, and which provides a greater area of contact with the polished rod for reducing the possibility of transducer slippage.